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ABSTRACT
INKJET PRINTING MULTIFUNCTIONAL CHROMATIC SENSORS AND
CHROMISM STUDY
by
Aide Wu
The thermochromism and chemochromism of polydiacetylene (PDAs)
and PDA/ZnO
nano composites have been systematically studied by attentuated
total reflection (ATR)-
Fourier transform infrared (FTIR), temperature-dependent Raman,
colorimetric (using
optical densitometry) and differential scanning calorimetry
(DSC). Reversibility of PDAs
has been enhanced by the formation of chelation between the
carboxylic groups on side
chain of diacetylene and Zn ion. The thermochromatic transition
temperature increases
with the concentration of ZnO
Thin films of polydiacetylene (PDAs) and PDA/ZnO nanocomposites
have been
successfully fabricated by inkjet printing both solution type
and suspension type ink.
Results suggest that PDA monomers are well-aligned and closely
packed following
printing. By modifying the particle size of PDA monomers or the
diacetylene/ZnO
particle size, reversible PDA ink with wider range of ZnO
concentration and longer shelf
life could be obtained by using water based ink. Also, with
inkjet printing technology,
thin film of PDA and PDA/ZnO composites could be deposited on
different substrate
materials, such as paper, Kapton and Mylar film.
In order to further study alkyl side chain effect on the
sensitivity of PDA, Density
Function Theory (DFT) simulation is conducted, and the results
show that the torsion of
C-C bond is closely related to the length of the alkyl side
chain.
-
INKJET PRINTING MULTIFUNCTIONAL CHROMATIC SENSORS AND
CHROMISM STUDY
by
Aide Wu
A Dissertation
Submitted to the Faculty of
New Jersey Institute of Technology
in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy in Materials Science and Engineering
Interdisciplinary Program in Materials Science and
Engineering
January 2015
-
Copyright © 2015 by Aide Wu
ALL RIGHTS RESERVED
.
-
APPROVAL PAGE
INKJET PRINTING MULTIFUNCTIONAL CHROMATIC SENSORS AND
CHROMISM STUDY
Aide Wu
Dr. John F. Federici, Dissertation Co-Advisor Date
Distinguished Professor of Physics, NJIT
Dr. Zafar Iqbal, Dissertation Co-Advisor Date
Research Professor of Chemistry and Environmental Science,
NJIT
Dr. Somenath Mitra, Committee Member Date
Distinguished Professor of Chemistry and Environmental Science,
NJIT
Dr. Robert B. Barat, Committee Member Date
Professor of Chemical Engineering, NJIT
Dr. Cristiano L. Dias, Committee Member Date
Assistant Professor of Physics, NJIT
-
BIOGRAPHICAL SKETCH
Author: Aide Wu
Degree: Doctor of Philosophy
Date: January 2015
Undergraduate and Graduate Education:
• Doctor of Philosophy in Materials Science and Engineering,New
Jersey Institute of Technology, Newark, NJ, 2015
• Master of Science in Materials Science and Engineering,Beijing
General Research Institute for Nonferrous Metals, P. R. China,
2010
• Bachelor of Science in Materials Science and
Engineering,Beijing University of Technology, Shanghai, P. R.
China, 2007
Major: Materials Science and Engineering
Publications and Patents:
Wu, A.; Beck, C.; Ying, Y.; Federici, J.; Iqbal, Z.
Thermochromism in Polydiacetylene -ZnO Nanocomposites. The Journal
of Physical Chemistry C 2013 , 117, 19593-19600.
Wu, A.; Gu, Y.; Beck, C.; Iqbal, Z.; Federici, J. F. Reversible
Chromatic SensorFabricated by Inkjet Printing TCDA-ZnO on a Paper
Substrate. Sens. Actuators B2014 , 193 , 10-18.
Wu, A.; Gu, Y.; Stavrou, C.; Kazerani, H.; Federici, J. F.;
Iqbal, Z. Inkjet printingcolorimetric controllable and reversible
poly-PCDA/ZnO composites. Sens.Actuators B 2014 , 203 ,
320-326.
Wu, A., Gu, Y.; Tian, H.; Federici, J. F.; Iqbal, Z. Effect of
Alkyl Chain Length onChemical Sensing of Polydi acetylene and
Polydiacetyl ene/ZnO N anocomposites.J. Colloid Polym. Sci. 2014 ,
DOI : 10.1007/s00396-014-3365-y.
Wu, A.; Gu, Y. Study on electrochemical performance of
carbon-coated LiFePO 4.Emerg. Mater. Res. 2013 , 2(3) ,
133-137.
iv
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v
Gu, Y.; Zhang, X.; Lu, S.; Zhao, T.; Jiang, D.; Yang, R.; Wu, A.
LiFePO4/C via Fluoride
Doping. Rare Metals 2012, 31(6), 573-577.
Patents:
Wu, A; Federici, J; Iqbal, Z. “PCDA/Zinc Oxide Nanocomposites
and Film Sensors”,
2014
Wu, A; Federici, J; Iqbal, Z. “Polydiacetylene and
Polydiacetylene/ZnO Nanocomposite
Chemical Sensors”, 2014
Wu, A; Gu, Y; Federici, J. “Systems and Method for
Environmentally Friendly Inkjet
Printing of Lithium Battery Cathode with Aqueous Binder”
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vi
This dissertation is dedicated to my parents and my wife
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vii
ACKNOWLEDGMENT
I would like to thank my dissertation co-advisors, Dr. John F.
Federici and Dr. Zafar
Iqbal, for their enduring support and supervision throughout my
research. Also, I would
like to thank Dr. Somenath Mitra, Dr. Robert B Barat and Dr.
Cristiano L. Dias for their
contributions to my research with their constructive
suggestions.
I would like to acknowledge the US Army Armament Research,
Development and
Engineering Center (ARDEC) at Picatinny Arsenal, NJ for
supporting this work
financially through Mr. James Zunino.
Lastly, I would like to thank my parents and my wife for their
infinite care and
support.
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viii
TABLE OF CONTENTS
Chapter Page
1 INTRODUCTION……............................………………..…………………………
1
1.1 Motivation of Polydiacetylene (PDA) Based Chromatic Sensors
Development. 1
1.2 Conjugated Polymers …………….…………………………………….…........ 2
1.3 Polydiacetylene Polymer ………….…………………………………….…....... 3
1.4 Inkjet Printing Technology ……….…………………………………….…....... 9
1.5 Inkjet Printing of PDA or PDA Monomers …………………………….….......
10
1.6 Significance and Benefits ……….…………………………………….…..........
11
2 THERMOCHROMISM IN POLYDIACETYLENE-ZINC OXIDE
NANOCOMPOSITES ……………………………….…………………….………. 13
2.1 Introduction ……...…………………………………………………………….. 13
2.2 Experimental Section ………………………………………………………….. 14
2.2.1 Materials …………...………………………………………………….... 14
2.2.2 Synthesis of Poly-TCDA-ZnO Nanocomposites ………………………..
15
2.2.3 Raman Spectroscopy ……………………………………………………. 15
2.2.4 ATR-FTIR Spectroscopy ……………………………………………….. 16
2.2.5 Optical Densitometry …………………………………………………… 16
2.2.6 Differential Scanning Calorimetry (DSC) ……………………………...
16
2.3 Results and Discussion ……………………………………………………….. 16
2.4 Conclusions …………………………………………………………………… 33
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ix
TABLE OF CONTENTS
(Continued)
Chapter Page
3 REVERSIBLE CHROMATIC SENSOR FABRICATED BY INKJET PRINTING
TCDA-ZINC OXIDE ON A PAPER SUBSTRATE ………………………………. 34
3.1 Introduction ………………………………...………………………………….. 34
3.2 Experimental Section ………………………………………………………….. 35
3.2.1 Materials ………………………………………………….......................
35
3.2.2 Preparation of TCDA and TCDA-ZnO Composites Ink
…………..….... 36
3.2.3 Design and Fabrication of Poly-TCDA Based Chromatic Sensor
……… 36
3.2.4 Synthesis of Poly-TCDA-ZnO Nanocomposites ………………………..
36
3.2.5 Material Characterization Techniques …………………………………..
37
3.3 Results and Discussion ………………………………………………………....
38
3.3.1 Feasibility of Inkjet Printing TCDA and TCDA-ZnO
Composites …….. 39
3.3.2 Raman and ATR-FTIR Spectroscopy of Poly-TCDA and
Poly-TCDA-
ZnO Composites ……………………………………………………....... 40
3.3.3 Temperature-Dependent Raman Spectroscopy of Poly-TCDA and
Poly-
TCDA-ZnO Composites ………………………………………………... 45
3.3.4 Differential Scanning Calorimetry (DSC) Measurements
……………… 48
3.3.5 Optical Densitometry …………………………………………………… 50
3.4 Conclusions ……………………………………………………………………. 56
4 INKJET PRINTING COLORIMETRIC CONTROLLABLE AND REVERSIBLE
POLY-PCDA/ZINC OXIDE …………………………………………………..…... 57
4.1 Introduction ……………………………………………………………………. 57
4.2 Experimental Section ………………………………………….………………. 58
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x
TABLE OF CONTENTS
(Continued)
Chapter Page
4.2.1 Materials …………………………………………………..……………. 58
4.2.2 Preparation of PCDA and PCDA/ZnO Composites Ink ………………..
58
4.2.3 Fabrication of Poly-PCDA and Poly-PCDA/ZnO Composites by
Inkjet
Printing .…………………………………………..…..............................
59
4.2.4 Material Characterizations ……………………………………………… 59
4.3 Results and Discussion ………………………………………………………… 60
4.3.1 Inkjet Printing of PCDA and PCDA/ZnO ……………………………… 60
4.3.2 Thermochromism in Poly-PCDA/ZnO Composites Fabricated by
Inkjet
Printing …………………………………………………………………. 64
4.3.3 Colorimetric Measurements …………………………………………….. 77
4.4 Conclusions ……………………………………………………………………. 80
5 EFFECT OF ALKYL CHAIN LENGTH ON CHEMICAL SENSING OF
POLYDIACETYLENE AND POLYDIACETYLENE/ZINC OXIDE
NANOCOMPOSITES ……………………………………………………………... 81
5.1 Introduction ……………………………………………………………………. 81
5.2 Experimental Section ………………………………………………………….. 82
5.2.1 Materials ………………………………………………………………... 82
5.2.2 Synthesis of PDA/ZnO Nanocomposites ……………………………….. 82
5.2.3 Material Characterizations ……………………………………………… 83
5.3 Results and Discussion ………………………………………………………… 84
5.3.1 ATR-FTIR Spectroscopy ……………………………………………………. 84
5.3.2 Raman Spectroscopy ……………………………………………………. 85
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xi
TABLE OF CONTENTS
(Continued)
Chapter Page
5.3.3 Density Functional Theory Simulations ………………………………...
92
5.3.4 RGB Measurements …………………………………………………….. 96
5.4 Conclusions ……………………………………………………………………. 98
6 DFT STUDY ON POLYDIACETYLENES AND THEIR DERIVATIVES ……… 99
6.1 Introduction ……………………………………………………………………. 99
6.2 Computational Details …………………………………………………………. 99
6.3 Results and Discussion ………………………………………………………… 100
6.3.1 Structures and Stabilities of PDAs ……………………………………… 100
6.3.2 Electronic Transition Energy in PDA ………………………………… 103
6.3.3 The Carbon Chain Conformation ……………………………………… 107
6.4 Conclusions ……………………………………………………………………. 110
7 SUMMARY ………………………………………………………………………... 111
REFERENCES ………………………………………………………………………... 112
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xii
LIST OF TABLES
Table Page
3.1 C≡C and C=C Raman Peak Frequencies in Pure Poly-TCDA and in
Poly-
TCDA-ZnO Nanocomposites in the Blue and Red Phases …....…………………
44
5.1 Polymer Backbone Raman Frequencies for Different PDAs and
Corresponding
PDA/ZnO Nanocomposites in the Blue Phase in the Presence of
Organic
Liquids …………………………………………………………………………... 88
5.2 C-C Torsion Angle on the PDA Backbones
.………...…....………………..…… 95
6.1 Cis- and Trans-forms and the Optimized Bond Length (Å) of
PDAs (n from 4 to
24)....……..………………………………………………………………………. 102
6.2 The Strongest Oscillator Strengths (f) and the Corresponding
Vertical Transition
Energies (λ) of Cis- and Trans-forms of
PDAs...................................................... 105
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xiii
LIST OF FIGURES
Figure Page
1.1 Chemical structures of various conjugated
polymers.………..…………………..
3
1.2 Schematic of the topological polymerization of diacetylene
under UV radiation .
4
2.1 ATR-FTIR spectra at room temperature of: a) Pure poly-TCDA
in the blue and
red phases; b) and c) Poly-TCDA and poly-TCDA/ZnO in the blue
phase for
two concentrations of ZnO between 700 and 3300 cm-1
and expanded in the 750
and 1800 cm-1
spectral range, respectively. Panel d) shows a computer-
generated approximate model of the chelate
proposed...…..………...…....……... 17
2.2 785 nm laser-excited Raman spectra of: a) blue and red
phases of poly-TCDA at
room temperature; b) blue phase of poly-TCDA and
poly-TCDA/ZnO
composites with three different ZnO concentrations at ambient
temperature. ...... 20
2.3 785 nm laser excited Raman spectra of pure poly-TCDA as a
function of: a)
Increasing temperature, and b) Decreasing temperature
…..…………………….. 23
2.4 785 nm laser excited Raman spectra of poly-TCDA/ZnO (5 wt%)
as a function
of: a) Increasing temperature, and b) Decreasing
temperature.…………………. 25
2.5 785 nm laser excited Raman spectra of poly-TCDA/ZnO (15 wt%)
as a function
of: a) Increasing temperature, and b) Decreasing temperature
....………............. 27
2.6 Temperature dependence on heating and cooling of the polymer
backbone C≡C
and C=C stretching mode frequencies of poly-TCDA and
poly-TCDA/ZnO
composites with different ZnO contents
.…...…………….................................... 28
2.7 Heating DSC scans for: a) TCDA monomer; b) poly-TCDA; c)
ZnO
nanopowder (
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xiv
LIST OF FIGURES
(Continued)
Figure Page
3.2 Digital photographs of inkjet printed TCDA: (a) TCDA monomer
before UV
radiation; (b) TCDA after UV radiation ………………………………………… 40
3.3 ATR-FTIR spectra at room temperature of: (a) Pure poly-TCDA
in the blue and
red phases; and (b) and (c) Poly-TCDA and poly-TCDA-ZnO in the
blue phase
between 700 and 3300 cm-1
and expanded in the 750 and 1900 cm-1
spectral
range ……………………………………………………………………………... 41
3.4 785 nm laser-excited Raman spectra of the inkjet printed
blue (bottom) and red
(top) phases of poly-TCDA at room temperature ………………………………..
43
3.5 Raman spectra of pure poly-TCDA and poly-TCDA-ZnO thin film
fabricated by
inkjet printing ……………………………………………………………………. 45
3.6 785 nm laser excited Raman spectra of pure poly-TCDA as a
function of: a)
Increasing temperature, and b) Decreasing temperature …………………………
46
3.7 785 nm laser excited Raman spectra of poly-TCDA-ZnO (2.5
wt%) as a
function of: a) Increasing temperature, and b) Decreasing
temperature ………... 47
3.8 C≡C and C=C stretching mode frequencies versus temperature
for inkjet
printed poly-TCDA and poly-TCDA-ZnO as a function of temperature
……….. 48
3.9 DSC heating scans for: a) TCDA, b) poly-TCDA, c) ZnO (
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xv
LIST OF FIGURES
(Continued)
Figure Page
4.3 Digital photographs of: (a) Inkjet printed PCDA/ZnO
composite on
conventional paper substrate, (b) Inkjet printed PCDA/ZnO
composite on
Kapton film substrate, (c) Poly-PCDA/ZnO at 25°C and (d)
Poly-PCDA/ZnO at
150°C ……………………………………………………………………………. 63
4.4 ATR-FTIR spectra at room temperature of: (a) Inkjet ptinted
poly-PCDA in the
blue and red phases; (b) and (c) Inkjet printed poly-PCDA and
poly-
PCDA/ZnO composites in the blue phase between 700 and 3000
cm-1
and
expanded in the 700 and 1900 cm-1
spectral range ……………………………… 64
4.5 785 nm laser-excited Raman spectra of: a) Blue (bottom) and
red (top) phases of
poly-PCDA at room temperature; b) Blue phase of poly-PCDA and
poly-
PCDA/ZnO composites with three different ZnO concentrations at
ambient
temperature; c) PCDA and PCDA/ZnO with three different ZnO
concentrations 67
4.6 785 nm laser excited Raman spectra of pure poly-PCDA as a
function of: a)
Increasing temperature, and b) Decreasing temperature …………………………
70
4.7 785 nm laser excited Raman spectra of pure poly-PCDA/ZnO (5
wt%) as a
function of: a) Increasing temperature, and b) Decreasing
temperature ………… 71
4.8 785 nm laser excited Raman spectra of pure poly-PCDA/ZnO (10
wt%) as a
function of: a) Increasing temperature, and b) Decreasing
temperature ………… 72
4.9 785 nm laser excited Raman spectra of pure poly-PCDA/ZnO (15
wt%) as a
function of: a) Increasing temperature, and b) Decreasing
temperature ………… 73
4.10 The polymer backbone C≡C and C=C stretching mode
frequencies of poly-
PCDA and poly-PCDA/ZnO composites with different ZnO content on
heating
and cooling ………………………………………………………………………. 75
4.11 Wavenumber specific vibration peaks as a function of ZnO
concentration (in
blue phase and red phase of poly-PCDA/ZnO): a) C≡C stretching
mode; b) C=C
stretching mode ………………………………………………………………….. 76
4.12 (a) Chromaticity versus temperature plots for poly-PCDA and
poly-PCDA/ZnO
composites of three different compositions; (b) Chromaticity of
poly-
PCDA/ZnO composites as a function of thermal cycle ………………………….
78
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xvi
LIST OF FIGURES
(Continued)
Figure Page
4.13 (a) Array of cropped photographic images of PCDA and
PCDA/ZnO
composites fabricated by inkjet printing on conventional paper
at different
temperatures; (b) Histogram of RGB values of the photographic
images
analyzed by software …………………………………………………………….. 79
5.1 ATR-FTIR spectra at room temperature in the blue phase of
poly-DCDA, poly-
PCDA and poly-TCDA, and their corresponding ZnO composites
…………….. 84
5.2 Raman spectra of poly-DCDA, poly-PCDA and poly-TCDA, and
their
corresponding ZnO nanocomposites in the blue phase at room
temperature …… 85
5.3 Raman spectra of poly-TCDA and poly TCDA/ZnO in the blue
form and in
different organic liquids …………………………………………………………. 89
5.4 Raman spectra of poly-PCDA and poly PCDA/ZnO in the blue
form and in
different organic liquids …………………………………………………………. 90
5.5 Raman spectra of poly-DCDA and poly DCDA/ZnO in the blue
form and in
different organic liquids ………………………………………………………… 91
5.6 Structures of simulated PDA segments: a) Poly-TCDA, b)
poly-PCDA, and c)
poly-DCDA ……………………………………………………………………… 93
5.7 Structure of the PDA segment used for the C-C torsion angle
study …………… 94
5.8 Potential energy curve as a function of torsion angle around
the central C-C
bond in cis-carbon with reoptimization of other geometrical
parameters as
discussed in the text ……………………………………………………………... 96
5.9 (Top panel) Array of cropped photographic images of PDAs and
PDA/ZnO
nanocomposites in selected organic liquids; and (Bottom panel)
Histogram of
RGB values of the photographic images analyzed by software
…………………. 97
6.1 HOMO and LUMO orbitals of PDAs backbones ………………………………..
103
6.2 HOM-OLUMO energy gaps (eV) of the cis- and trans-form
isomers vs the
number of carbon atoms …………………………………………………………. 104
6.3 Fermi energy level of cis- and trans- carbon chain vs the
number of carbon
atoms …………………………………………………………………………….. 105
6.4 Vertical transition energy vs Chain carbon number of
polydiacetylene: (a) cis-
form carbon chain; (b) trans-form carbon chain …………………………………
106
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xvii
LIST OF FIGURES
(Continued)
Figure Page
6.5 Snap shot of structures for the simulations in series1 and
series 2 (side chain
with 5 carbon atoms) …………………………………………………………….. 108
6.6 Torsion angle of C-C on the backbone of PDA vs side chain
length …………… 109
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1
CHAPTER 1
INTRODUCTION
1.1 Motivation of Polydiacetylene (PDA) Based Chromatic Sensors
Development
Chromatic sensors play a very important role in sensing. The
main advantage of chromatic
sensor is that they provide a visual color indication of sensing
results without the need to
convert the sensing results to an electronic or numeric signal
for further processing1.
Essentially, a visible color change in the sensing material, due
to some external stimulus
such as temperature, is readily recognized. As a chromatic
sensor material,
polydiacetylenes have been widely studied and used, because they
can respond to different
stimuli, such as mechanical stress, thermal stress, chemical
stress, and so on2-6
. However,
there are two main problems with directly depositing uniform and
functionalized layers of
PDA aggregates during the coating process:
(a) The PDA coating has a lower density if it is directly coated
onto a substrate.
This is a consequence of PDA’s long chain structure and lack of
chain
orientation.
(b) PDA is not soluble in most solvents and PDA aggregates are
very hard to
disperse. Those two factors cause PDA to distribute unevenly on
substrates
resulting in a rough coating surface.
The poor coating quality with PDA aggregates has limited the
application of PDAs.
The advent of topochemical methods in synthesis of PDA has not
only successfully solved
the aggregation problem, but also has made PDA designable for
different applications7-10
.
With different pendent site groups, PDA can be formulated for
different sensitivity ranges
for a specific stimulus. With the help of solid state
polymerization methods, nano-sized and
PDA/inorganic composites have been synthesized and reported
11-16
. Methods that have
been used for fabrication of thin polymer films are spin
coating, self-assembly and
-
2
Langmuir-Blodgett (LB) or Langmuir–Schaefer methods. Inkjet
printing method has been
given more attention in polymer thin film fabrication17-20
.
1.2 Conjugated Polymers
Polymers containing alternating saturated/unsaturated bond and
delocalized π- electrons
through their backbones, are often referred as a conjugated
polymer (CP) or conducting
polymers. Conjugated polymers are very unique polymers because
of their extended
π-conjugated system on the backbone. The extended π-bonds
contain continuous
delocalized electrons which give rise to the unique optical and
electronic properties. As a
result, these electro-active polymers are used in a variety of
applications including
field-effect transistors, polymer actuators, light-emitting
materials, sensors and solar cells.
The practical value of conjugated polymers was recognized by
Nobel Prize in chemistry in
2000. Numerous CPs have been investigated and some examples of
typical CPs are
represented in Figure 1.1. Conjugated polymer (CP) systems are
very attractive in a sensor
design because their absorption and emission properties are very
sensitive to
environmental perturbations15,21-25
. CPs-based sensor systems typically compared to
conventional small molecular sensors systems by their potentials
for signal amplification
when subjected to external stimuli26-28
. Accordignly, a variety of conjugated polymers such
as polythiophenes29
, polyanilines30,31
, poly(phenylene ethynylenes)32,33
, polyacetylenes34
,
and polydiacetylenes35
, have been studied as sensor matrices.
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3
Figure 1.1 Chemical structures of various conjugated
polymers.
1.3 Polydiacetylene Polymer
Since the first reports on polydiacetylene (PDA) synthesis
appeared at the end of the 1960s
by Wegner et al.36,37
, these molecules have captured the imagination of scientists
and
technologists alike due to their unique chromatic properties.
Specifically, it has been
shown that certain diacetylene monomers can be aligned in
solutions and polymerized
through ultraviolet (UV) irradiation, producing a conjugated PDA
network38,39
(Figure
1.2). The unique feature of PDA systems has been the observation
that the conjugated PDA
networks often absorb light in the visible spectral region,
thereby exhibiting color, in most
cases blue40,41
. Moreover, conjugated PDA can undergo phase changes, induced by
varied
environmental stimuli, leading to dramatic colorimetric
transformations that are visible to
the naked eye. Another attractive feature of PDA systems in the
context of sensing
applications has been the fluorescence properties; blue phase
PDA is non-fluorescence
while the red-phase configuration exhibits high fluorescence
with minimal bleaching42-44
.
Beside the intriguing chromatic properties of PDA, the diverse
physical
configurations of the PDA have attracted broad research
interest. PDA systems have been
shown to organize in vesicles45,46
, Langmuir monolayers47-49
, self-assembled films50,51
, and
single crystals52,53
. PDA has been also assembled as components within other
“host”
-
4
matrixes, including inorganic matrixes54-56
, other polymers57,58
, and even living cells59
.
Remarkably, it has been shown that PDA generally retains its
chromatic properties in these
configurations, thus opening the way to construction of varied
sensing assemblies.
Basic Molecular Properties of PDAs
Figure 1.2 Schematic of the topological polymerization of
diacetylene under UV radiation.
The unique chromatic properties of PDA systems arise from the
molecular
properties of the polymer. PDA is formed through 1,4 addition of
aligned diacetylenic
monomers, initiated by ultraviolet (UV) irradiation (Figure
1.2). The diacetylene
monomers do not absorb light in the visible region, while
polydiacetylene appears intense
blue (absorption peak at around 650 nm) due to electron
delocalization within the linear
p-conjugated framework, and corresponding to a π–π*
transition.
As indicated above, the colorimetric transformations of PDA,
induced by a variety
of external stimuli, have likely been the most interesting and
technologically-attractive
feature of PDA systems. The significant shift of the absorption
peak from around 640 nm
-
5
(the blue phase) to around 500 nm (the red phase) is ascribed to
disruption of the
conjugated network, resulting in shorter electronic
delocalization lengths. The red phase of
PDA is accompanied by intense fluorescence, which further
exhibits negligible bleaching,
contributing to utilization of the fluorescence properties in
varied sensing applications.
Despite decades of studies, elucidating the exact mechanisms
responsible for the chromatic
transformations of PDA has not been fully accomplished. It has
been recognized that the
shifts in spectral absorbance are closely linked to structural
modifications of the
conjugated polymer framework49
. Early models accounting for the spectral/structural
modulations proposed transformation of the polymer backbone from
the ene–yne to a
butatriene conformation60
. Recent crystallographic and theoretical investigations
have
illuminated intimate structural aspects pertaining to the
chromatic properties. In particular,
it has been established that the pendant side-chains of PDA play
a prominent role in
affecting the chromatic transformations. The interactions
between the functional groups of
the side-chains are believed to significantly affect the overall
conformation of the polymer
chain, primarily rotations around the C–C bonds affecting the
planarity of the backbone
and concomitant overlap between adjacent π orbitals 49
. Indeed, theoretical calculations
suggested that even rotation of a few degrees of the side-groups
around the C–C bond
would give rise to a significant change of the π-orbital overlap
and resultant blue-red
transition61
.
The realization that PDA side-chains exhibit significant effects
upon the chromatic
properties of the polymer has led to intense research aiming to
modulate PDA spectral
response through synthetic modifications of side-chain
functional groups. Efforts have
been directed, for example, to alter the crystal packing of the
individual monomers (the
-
6
essential precondition to photo-polymerization) and the
resultant linear polymer chains
(via side-chain modification). A notable consequence of the
close links between crystal
packing and pendant side-chain orientation is the achievement of
color reversibility. While
most of the early work on supramolecular PDA assemblies
demonstrated irreversible color
transformations, there have been an increasing number of reports
depicting color-change
reversibility via chemical modification of the PDA
side-chains62-64
, thus altering the
molecular packing and topochemical transformations within the
polymer modules65,66
.
Synthetic pathway to PDAs
Early work in the field has mostly focused upon the “standard”
diacetylene monomers
10,12-tricosadynoic acid and 5,7-pentacosadiynoic acid. These
monomers, currently
commercially available, can be aligned in aqueous solutions and
the hydrogen bond
network maintained among the carboxylic headgroups enable the
occurrence of ene–yne
transformations and formation of the polymerized conjugated
backbone system 67
.
Recent years have witnessed a proliferation of synthesis schemes
producing novel
diacetylene monomeric units. For example, peptide–diacetylene
monomers in which the
diacetylene backbone is flanked by peptide moieties have been
reported68
. And the PDA–
peptide conjugates have been synthesized, bestowing interesting
properties to the resultant
materials1,69-71
. Vesicles comprising a PDA–histidine derivative and
PDA–pentalysine
which further contained a fluorescent moiety, constituted
vehicles for binding and
detection of lipopolysaccharides (LPS) – the prominent
recognition units displayed on
bacterial surfaces72
. Specifically, the organized positively-charged amino residues
in the
synthetically-modulated PDA vesicles mimicked the recognition
surface of polymyxin-B,
-
7
a natural antibiotic which specifically binds to LPS primarily
through electrostatic
attraction73
.
Monomeric diacetylene units have also been derivatized with
non-peptidic
residues. An assembly of porous “molecular columns” enabled
through the polymerization
process “diacetylene macrocycle” units have been reported 74
. In essence, the polymerized
diacetylene network in this case provided the “scaffolding” for
the columns, rather than the
means for optical/spectroscopic transformations. A systematic
study has recently
investigated the crystalline organizations of diacetylene
monomers functionalized with
different phenyl-containing units75
, revealing that despite the bulky phenyl-substituted
headgroups, aromatic perfluorophenyl–phenyl interactions
facilitated efficient
polymerization and formation of the conjugated PDA backbone.
Indeed, while diacetylene
monomers can be readily manipulated via diverse synthetic
routes, in many cases the
resultant molecules do not undergo topotactic polymerization to
the polymer phase since
the structural modifications disrupt the monomer alignment
essential for the ene–yne
transformation76,77
.
Attaining reversibility of PDA chromatic changes has been among
the most
remarkable achievements in this field. In that regard, synthetic
progress has often gone
beyond establishing a firm understanding of the molecular
mechanisms pertaining to color
reversibility in PDA systems. It is generally accepted that
reorganization of the hydrogen
bond network through synthetic manipulations of the diacetylene
headgroups is the core
factor making possible reversible blue-red transformations of
PDA. Accordingly, most
reversible PDA systems have employed varied schemes for
manipulating the polar
head-groups of the polymer.
-
8
Almost all examples of reversible PDA-based systems have focused
upon
thermally-induced transformations, i.e. blue-red changes brought
about by heating, while
the reversible red blue transformation occurring following
cooling. Among the diacetylene
units shown to affect color reversibility following
polymerization were monomers
displaying 2,2,2- trifluoro-N-(4-hydroxyphenyl) acetamide78
, 3-carboxypropylpentacosa-
10,12-diynamide79
, azo chromophore-functionalized diacetylene80
, secondary amine
salts81
, naphthylmethylammonium carboxylate and non-polar benzyl
moieties82, 83
.
Intriguing reversible thermochromism has been demonstrated in a
supramolecular
system in which PDA was not derivatized (i.e. the conventional
10,12-pentacosadiynoic
acid has been used)84
. In that system, temperature-induced reversible color
transitions were
traced to a hierarchical organization in which the PDA domains
were encapsulated within a
poly (vinylpyrolidone) (PVP) matrix. The intercalation of PVP
domains within the PDA
framework was the likely factor enabling reorientation of the
PDA head groups affecting
reversibility of the conjugated polymer network length (and
consequent reversible
chromatic transitions).
PDA-based composite materials
Composite materials comprising PDA mixed with or coupled to
other molecular species
have been pursued, yielding in many cases advanced materials
exhibiting interesting
properties. PDA–carbon nanotubes (CNTs) are a case in
point85,86
. In such systems, the
surfaces of single-walled CNTs (SWCNTs) have been used as a
“template” upon which
organization and polymerization of the diacetylene units
occurred. The resultant composite
materials exhibited intriguing properties. Through interactions
with the pendant
side-chains of the polymer “ring-shaped” polydiacetylene
structures formed upon the
-
9
SWCNT surface were capable of solubilizing and stabilizing
highly hydrophobic
substances, such as membrane proteins and dyes85
. Accordingly, such PDA–SWCNT
constructs might find uses as “smart detergents” in cosmetics,
membrane proteins structure
determination, and others. Also, it has been noticed that CNTs
not only provided a physical
framework upon which polymerization could be carried out, but
also constituted a source
of fluorescent energy87
, enable the application of this PDA composite in cellular
imaging
applications. A conceptually-similar PDA–nanostructure system
has utilized magnetic
nanoparticles (NPs) as a template for assembly of the chromatic
polymer88
, the magnetic
particles not only enabled the topotactic polymerization of the
monomers into the extended
polymer network, but also enable the blue-red transformation
could be induced by a
magnetic field.
Another major route in PDA technology development is conjugation
of PDA with
inorganic, porous materials. In general, porous materials
constitute attractive targets for
practical applications involving PDA. This is due to the large
(internal) surface areas of
porous matrixes useful for immobilization of high polymer
concentration, and
transparency – making possible exploiting the
optical/spectroscopic properties of PDA
within the inorganic framework89
. Also, modulation of the diacetylene headgroups should
not be ignored, and it has led to preparation of self-assembled
species displaying
remarkable structural and functional properties, realizing more
PDA composite materials.
By modifying the headgroup of PDA, PDA could be endowed with
useful biological
imaging functionalities90
.
-
10
1.4 Inkjet Printing Technology
Inkjet printing is one kind of fabrication method that enables
deposition of materials into
various patterns on different types of substrate and it has been
widely applied to thin film
fabrication. Metallic, polymer and bio materials are examples of
materials that have been
printed by inkjet printing methods. Because of its
drop-on-demand feature, inkjet printing
has evolved from text and graphic processing to its adaptation
as a rapid manufacturing
technique. Compared with other fabrication methods such as
photolithography in
fabricating micro and nano electronic devices, inkjet printing
has the following
advantages91-93
:
Non-contact and low cost method of fabrication
Ability to deposit precise amount of materials in a rapid
way
Ability to print on specific locations which is controlled by
computer
Low temperature processing with no need for a vacuum
Compatibility with various substrates
Multilayer structured design
1.5 Inkjet Printing of PDA or PDA Monomers
Due to the features of inkjet printing, materials used for
inkjet printing either have to be
soluble or the particle size must be much smaller than the
nozzle openings of the printing
heads. So far, there is no report on directly inkjet printing
PDAs. However, this is not
surprising because the aggregate of PDAs is too big to fit
through the nozzles of printing
head.
-
11
PDA monomers rather than PDA aggregates are possible for inkjet
printing,
because with proper processing and treatment the monomer sizes
will be smaller than the
nozzle openings. However, inkjet printing of PDA monomers has
rarely been reported.
There is only a few reports on fabrication of PDA films by using
inkjet printing. Yoon et al.
used the inkjet printing method to print PCDA with different
pendant site groups (PCDA,
PCDA-AEE, and PCDA-mBzA). By adding nonionic surfactant Brij78,
the polymerized
PCDA-mBzA shows very good thermal response. Thermal stimulus
caused color-change
reversibility and can respond to electrical stimulus as well
94
. In addition, PCDA-mBzA/
Brij78 has be reported by U. Zschieschang et al. as a
counterfeit-proof ink for banknotes 95
.
1.6 Significance and Benefits
There are many scientific papers reported utilizing
Polydiactylene (PDA) as a chromatic
sensor by fabrication methods such as spin-coating,
nano-assembly, and other methods,
but only a few report sensor fabrication by inkjet printing.
Also, only inkjet printing of
PCDA based suspension has been reported. Ink-jet printing of
other PDA monomers has
not been reported. In addition, synthesizing PDA/ZnO composites
by using inkjet printing
is novel and promising.
The major benefits of proposed work are as follow:
Chromatic sensors, by their very nature of indicating a change
in a stimulus (such as temperature) through a visible color change,
enable quick and visual
interpretations of the sensor’s state.
The ability to ink-jet print chromatic sensor interjects a low
cost method and simplicity of fabrication on multiple and
potentially flexible substrates.
Because PDA can change color in response to thermal, chemical,
mechanical
stimulus, PDA based chromatic sensors can be used as temperature
indicators, chemical
-
12
agent detectors, and/or circuit protection devices. For some
specific practical applications
of a PDA film sensor, it could be inkjet printed onto a decal
which could then be attached to
an ammunition box to indicate the temperature. It is a
well-known issue with the storage of
explosives that the explosives will decompose during storage at
elevated temperatures.
Another simple example is a PDA sensor printed on conventional
paper substrate which
could be used as a disposal chemical sensor in food
inspections.
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13
CHAPTER 2
THERMOCHROMISM IN POLYDIACETYLENE-ZINC OXIDE
NANOCOMPOSITES
2.1 Introduction
Chromatic sensors play an important role in different types of
sensing. The main advantage
of a chromatic sensor is that it provides visual color
indication without the need to convert
to a digital signal 96
. Polydiacetylenes (PDAs), have been widely studied as a
chromatic
sensor material because they can respond to mechanical,
temperature and chemical stimuli
2-6. Solid state topotactic photo-polymerization of diacetylene
monomers by exposure to
UV or γ-radiation and subsequent thermochromism in closely
packed and uniformly
ordered thin films of various PDAs are well known 97
and have been widely studied for
temperature-sensing applications.
PDAs have a one-dimensional conjugated backbone with a strong π
to π*
absorption band in the red spectral region of the optical
spectrum which gives rise to an
intense blue color in the polymer. The blue phase undergoes a
heat-induced thermochromic
transition observed in many PDAs to a red phase. The blue to red
chromatic transition is
either irreversible or reversible under heating and cooling
cycles depending on the
chemical structure and interactions on the side chains of the
PDA. In the blue phase, the
strain induced by hydrogen bonding at the head groups leads to
an increase in π- electron
conjugation length. However, when hydrogen bonding interactions
are disrupted by heat,
the side group strain is released leading to twisting of the π-
electron orbitals, decrease of
π-electron conjugation98
and concomitant transition to a red phase. The red phase can
rapidly reverse back to the blue phase on cooling when
interactions due to: (a) Strong head
-
14
aromatic groups 99
, (b) Ionic moieties 100
, and (c) Covalent bond 54,101
, enhanced hydrogen
84,102-106 and multibonding bonding at the head groups
107-109 are present in the PDA
structures. The red phase is irreversible when the head group
interactions cannot be
restored on cooling. These PDAs are therefore either
irreversible or reversible sensors.
PDAs prepared from 10, 12-pentacosadiynoic acid (PCDA) and
10,12-docosadiynedioic acid (DCDA) have been widely investigated
110-112
, but little
attention has been given to the related but important monomer
with a shorter hydrocarbon
side chain, CH3(CH2)9-C≡C-C≡C-(CH2 )7CH2COOH (10,
12-tricosadiynoic acid, TCDA).
Previous work performed in this group (Patlolla et al 113
) on poly-PCDA-metal oxide
nanocomposites provided a broad understanding of the changes in
chromatic properties of
the nanocomposites relative to those of pure PCDA. Here a more
detailed investigation is
carried out using Raman spectroscopy, DSC and colorimetry using
optical densitometry as
a function of temperature on poly-TCDA and poly-TCDA/ZnO
nanocomposites, together
with an ATR-FTIR study at ambient temperature to extract a
molecular level
understanding of poly-TCDA/ZnO nanocomposite formation.
2.2 Experimental Section
2.2.1 Materials
TCDA was purchased from GFS Chemicals and nanocrystalline ZnO
(
-
15
2.2.2 Synthesis of Poly-TCDA-ZnO Nanocomposites
Poly-TCDA/ZnO suspensions were prepared by suspending different
amounts of ZnO (5
wt%, 10 wt%, 15 wt%) in solution of the TCDA monomer (1 mM ) in
chloroform. The
suspension contained in a beaker was sonicated in a water bath
at room temperature for 30
min and dried at 40 °C with magnetic stirring for 8 hours. The
magnetic stirring was
stopped after the liposome state was achieved. The pure TCDA and
TCDA composites
were polymerized to the blue phase of poly-TCDA and
poly-PCDA-ZnO composite by
irradiating with a 254 nm wavelength UV source. Powders of the
blue phase composite
were obtained by scraping from the beaker and grinding into a
fine powder. Red phase
composite powders and films were similarly produced after
heating the blue phase to above
the thermochromic transition temperature.
2.2.3 Raman Spectroscopy
Raman spectra at room temperature were obtained primarily using
a Mesophotonics
Raman spectrometer with 785 nm laser excitation.
Temperature-dependent Raman
measurements were carried out with an EZRaman LE Raman Analyzer
system from
Optronics using 785 nm laser excitation coupled to a Leica
optical microscope. The
spectrometer was calibrated using silicon wafer and diamond
powder standards to a
frequency accuracy of 1 cm-1
. The variable temperature optical stage used is from Linkam
Scientific Instruments Ltd. Thick films for the Raman
measurements were prepared by
mixing suspensions of TCDA with certain amount of ZnO, using
chloroform as the
suspension medium. After drying and 254 nm uv-radiation, the
polymerized dry powder of
poly-TCDA and poly-TCDA/ZnO were measured on a silicon wafer
substrate.
-
16
2.2.4 ATR-FTIR Spectroscopy
Fourier Transform Infrared (FTIR) was carried out using a
Nicolet ThermoElectron FTIR
560 spectrometer with a MIRacle attenuated total reflectance
(ATR) platform assembly
and a Ge plate.
2.2.5 Optical Densitometry
Chromaticity, which is a quantitative measure of the vividness
or dullness of a color (or
how close the color is to either the gray or pure hue) was
measured directly on thin film and
coated samples using an X-Rite 518 optical densitometer as the
samples were heated on a
temperature-controlled hot plate.
2.2.6 Differential Scanning Calorimetry (DSC)
A Mettler Toledo DSC instrument (Mettle-Toledo Inc. Columbus,
OH, USA) with a FP90
central processor was used to obtain the DSC data of 10 mg of
precursor, polymer and
composite samples wrapped in a small disk with aluminum foil
using
heating/cooling/heating cycles in the temperature range from
25°C to 300 °C at a rate of
10°C min-1
.
2.3 Results and Discussion
Attentuated Total Reflection (ATR)-Fourier Transform Infrared
(FTIR) spectroscopy at
room temperature in both the red and blue phases for pure
poly-TCDA and for the blue
phase in poly-TCDA/ZnO together with Raman spectroscopy as a
function of temperature
for poly-TCDA and poly-TCDA/ZnO provide details about the
molecular structural
changes around the chromatic transition and molecular
interactions on nanocomposite
-
17
formation. The thermal and colorimetric changes as a function of
temperature at these
transitions are investigated further by DSC and optical
densitometry, respectively.
(a)
(b)
Figure 2.1 ATR-FTIR spectra at room temperature of: (a) Pure
poly-TCDA in the blue and
red phases; (b) Poly-TCDA and poly-TCDA/ZnO in the blue phase
for two concentrations
of ZnO between 700 and 3300 cm-1
spectral range, respectively.
-
18
(c)
(d)
Figure 2.1 ATR-FTIR spectra at room temperature of: (c)
Poly-TCDA and
poly-TCDA/ZnO in the blue phase for two concentrations of ZnO
expanded in the 750 and
1800 cm-1
spectral range, respectively; Panel (d) shows a
computer-generated approximate
model of the chelate proposed. (Continued)
-
19
Figure 2.1a shows the ATR-FTIR spectra of poly-TCDA in its blue
and red phases,
and Figures 2.1b and 1c show the spectra of poly-TCDA and
poly-TCDA/ZnO in the 700
to 3500 cm-1
and expanded in the 700 to 1900 cm-1
regions, respectively. Lines at 2920 and
2847cm-1
are assigned to the asymmetric and symmetric stretching
vibrations,
respectively, of the CH2 groups on the side chains, and those at
1463, 1417 and 1694 cm-1
can be attributed to the CH2 scissoring and hydrogen-bonded
carbonyl C=O stretching
vibrations, respectively. On comparing the FTIR spectra of pure
poly-TCDA with that of
poly-TCDA/ZnO shown in Figures 2.1b and 2.1c, it is observed
that a relatively strong new
line appears at 1540 cm-1
in the spectrum of poly-TCDA/ZnO together with a concomitant
decrease in intensity of the C=O stretching line at 1694
cm-1
. The 1540 cm-1
line can be
assigned to an asymmetric COO− stretching vibration and its
presence in the spectra
together with a corresponding decrease in the intensity of the
C=O line suggests that a
chelate between neighboring side chain -COOH head groups of
poly-TCDA and Zn2+
ions
from ZnO is formed (see computer-generated approximate model in
Figure 2.1d). This
chemical interaction between ZnO and poly-TCDA, dependent on the
ionicity of the Zn-O
bond is likely to cause the high temperature red phase to
reverse back to the blue phase on
cooling 114
in poly-TCDA/ZnO composites.
-
20
(a)
(b)
Figure 2.2 785 nm laser-excited Raman spectra of: (a) blue and
red phases of poly-TCDA
at room temperature; (b) blue phase of poly-TCDA and
poly-TCDA/ZnO composites with
three different ZnO concentrations at ambient temperature.
Raman scattering due to the molecular vibrational modes of the
conjugated
polymer backbone are expected to be primarily resonance-enhanced
for excitation using
780 nm laser radiation. From the Raman spectra in Figure 2.2a
for pure poly-TCDA, two
intense lines at 2083 cm-1
and 1455 cm-1
are observed at room temperature in the blue
-
21
phase, which can be definitively assigned to the C≡C and C=C
stretching modes of the
polymer backbone, respectively. Note that the C=C stretching
mode is close in frequency
to a line at 1463 cm-1
assigned to a side chain CH2 deformation mode observed in
the
ATR-FTIR spectra in Figure 2.1. In the red phase the room
temperature C≡C and C=C
stretching vibration frequencies at 2114 cm-1
and 1516 cm-1
, respectively, increase due to
the irreversible stress on the polymer backbone due to
dissociation of the head group
hydrogen bonds in the red phase. The line intensities in the red
phase are lower because of
decreased resonance interaction with the polymer backbone. This
decrease in resonance
interaction with the polymer backbone in the red phase was not
evident in the Raman
spectrum of the red phase of PCDA25
and is likely to be due to the fact that the hydrocarbon
side chain is longer in PCDA. The Raman lines at frequencies
below that of the C=C
stretching mode can be assigned to Raman-active deformation and
C-C stretching motions
of the conjugated polymer backbone mixed with hydrocarbon chain
deformation modes.
The triplet of lines around 1250 cm-1
and the line at 690 cm-1
in the blue phase are relatively
intense as a result of resonance enhancement due to mixing of
the backbone C-C stretching
and deformation modes.
Figure 2.2b shows the Raman spectrum of pure poly-TCDA in the
blue phase
compared with the blue phase spectra of poly-TCDA/ZnO
composites. From Figure 2.2b,
it is evident that a very weak line at 2257 cm-1
in the C≡C stretching mode region of
poly-TCDA increases in intensity in the composite. By contrast,
a relatively weak line in
the C=C region at 1516 cm-1
in the blue phase due to a red phase impurity disappears on
composite formation. The line at 2257 cm-1
can be assigned to a diyne defect formed on the
backbone due to the chemical interaction between TCDA and ZnO
114
. However, the
-
22
intensity of this line appears to saturate at low ZnO
concentration and does not increase
with increasing ZnO. Another interesting feature in Figure 2.2b
which is consistent with
the chemical interaction of poly-TCDA with ZnO is that the line
at 690 cm-1
and the triplet
of lines at 1250 cm-1
assigned above to largely polymer backbone modes, show
substantial
increase in intensity in the composite phase.
Raman spectra under heating and cooling cycles in the 25 °C to
150 °C temperature
range for poly-TCDA and poly-TCDA/ZnO at different ZnO
concentrations are shown in
Figures 2.3-2.5. The Raman data were taken in steps of 10°C from
30 °C to 150 °C and also
recorded in 10 °C steps during the cool down to room
temperature. Figure 2.3 displays the
Raman spectra of poly-TCDA with increasing temperature to 150 °C
followed by cooling
from 140 °C to 30 °C. From the heat-up spectra in Figure 2.3a,
it is can be observed that the
backbone stretching and deformation lines in the blue phase
decrease in intensity with
increasing temperature as the sample goes to the red phase
consistent with the fact that
resonance-enhancement is weaker in the red phase as discussed
above. The weak line at
1516 cm-1
assigned to a red phase impurity in the blue phase grows in
intensity and
becomes the predominant C=C backbone stretching mode in the red
phase. From Figure
2.3b it is evident that the spectrum remains essentially
unchanged on cooling consistent
with the irreversibility of the red phase of poly-TCDA.
-
23
(a)
(b)
Figure 2.3 785 nm laser excited Raman spectra of pure poly-TCDA
as a function of:
(a) Increasing temperature, and (b) Decreasing temperature.
The heating and cooling Raman spectra of poly-TCDA/ZnO with the
ZnO content
at 5 wt% are shown in Figure 2.4. By contrast with the variable
temperature spectra for
pure poly-TCDA, in Figure 2.4a, a broad scattering band centered
near 690 cm-1
appears
reproducibly in the spectra with increasing intensity as the
temperature approaches and
goes above the ca. 120 °C melting transition of the ZnO
composites observed in the DSC
data (see discussion below and Figure 2.7). Note that the broad
scattering feature appears
-
24
below the melting transition temperature and increases in
intensity above 120 °C. It can be
tentatively assigned to light scattering from an amorphous
network of the the
poly-TCDA/ZnO complex. The scattering is not seen at higher ZnO
concentrations as
discussed below and it is also not observed in
poly-PCDA/ZnO113
at all concentrations of
ZnO probably because the diffusional motions of the longer
hydrocarbon side chain in
poly-PCDA compared with poly-TCDA prevents the formation of the
amorphous network.
The intensity from the amorphous network shows a small decrease
on cooling through the
melting temperature down to room temperature in Figure 2.4b.
Moreover, the features of
the spectra in Figure 2.4b show that the red phase of the
composite with 5% by weight of
ZnO converts only partially back to the blue phase on
cooling.
-
25
(a)
(b)
Figure 2.4 785 nm laser excited Raman spectra of poly-TCDA/ZnO
(5 wt%) as a function
of: (a) Increasing temperature, and (b) Decreasing
temperature.
Figure 2.5 shows the heating and cooling Raman spectra of
poly-TCDA/ZnO (15
wt%). Similar heating and cooling Raman spectra (not shown here)
were observed for
poly-TCDA/ZnO (10 wt%). Broad scattering due to amorphous
poly-TCDA/ZnO at these
higher ZnO concentrations are not observed (Figures 2.5a and
2.5b). Also, the red phase
spectrum changes rapidly back to that of the blue phase on
cooling. The Raman frequencies
-
26
of the C≡C and C=C backbone stretching vibrations of pure
poly-TCDA, poly-TCDA with
5wt%, 10 wt% and15 wt % of ZnO below 100 nm in size as a
function of heating and
cooling cycles are plotted as a function of temperature in
Figure 2.6. Note that the
frequency upshift in the red phase decreases with increasing ZnO
content suggesting that
the stress on the polymer backbone is lowered due to chelation
of ZnO with the head group
of poly-TCDA to make the chromatic transition reversible. The
plots in Figure 2.6 of the
Raman-active C≡C and C=C backbone stretching frequencies as a
function of temperature
cycling indicate increases in frequencies at the chromatic blue
to red transition at 70 °C on
heating for pure poly-TCDA and near 120 °C for the poly-TCDA/ZnO
composites. For
poly-TCDA/ZnO (5 wt%), the slight upshift of frequency of the
C=C and C≡C modes at
70°C could be due to non-chelated TCDA monomer. The frequency
upshift at 130 °C in
the composites is due to chelate formation between TCDA and
ZnO.
-
27
(a)
(b)
Figure 2.5 785 nm laser excited Raman spectra of poly-TCDA/ZnO
(15 wt%) as a
function of: (a) Increasing temperature, and (b) Decreasing
temperature.
-
28
Figure 2.6 Temperature dependence on heating and cooling of the
polymer backbone C≡C
and C=C stretching mode frequencies of poly-TCDA and
poly-TCDA/ZnO composites
with different ZnO contents.
Differential scanning calorimetry (DSC) measurements provide
further
understanding of the nature of TCDA/ poly-TCDA/ZnO interactions.
DSC data were
obtained for pure TCDA monomer, poly-TCDA, and poly-TCDA/ZnO, at
heating and
cooling rates of 10 °C min-1
between 25 °C and 300 °C. The heating scan for pure TCDA in
Figure 2.7a shows an endothermic peak at 61 °C due to melting.
On cooling (scan not
shown here) down-shifted exothermic crystallization peaks at 59
°C due to hysteresis are
observed. The heating scan for poly-TCDA in Figure 2.7b shows an
endothermic peak at
61 °C due to melting of the unpolymerized monomer. A broad
endotherm with a shoulder
at 154 °C and a peak at 190 °C are assigned to the melting of
poly-TCDA. On cooling (scan
-
29
not shown), polymer crystallization is indicated by broad
exothermic features at 159 °C
and 194 °C which are upshifted due to hysteresis relative to the
corresponding endothermic
melting peaks. Crystallization of unpolymerized monomer is not
observed during the
cooling cycle probably due to loss of the monomer by sublimation
during thermal cycling.
The heating scans for TCDA-ZnO nanocomposites in Figure 2.7d
show endotherm around
57 °C due to unpolymerized monomer and a new endothermic feature
at around 137 °C due
to melting of the monomer modified by chelate formation with ZnO
discussed above,
which coincides that fact that no endothermic feature of ZnO
(Figure 2.7c) is observed in
the DCS data of composites. It is also seen from Figure 2.7d
that with the increase of ZnO
content, the endotherm due to TCDA becomes weaker and the peak
shifts to higher
temperature indicating that the chelate between ZnO and head
group –COOH becomes
stronger because more chelate formation can occur with
increasing ZnO content. This is
consistent with the FTIR, Raman and DSC data discussed above
suggesting an interaction
of ZnO particles with the head group of the polymer side-chain
to form a chelate which can
be schematically written as: Zn2+
(COO−)2. In pure poly-TCDA, heating causes an
irreversible stress on the polymer backbone due to the
dissociation of hydrogen bonds
between the side chain head groups to form the red phase. In the
presence of ZnO, chelate
formation results in release of strain on cooling and reversal
back to the blue phase.
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30
Figure 2.7 Heating DSC scans for: (a) TCDA monomer; (b)
poly-TCDA; (c) ZnO
nanopowder (
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31
cycles from 25 °C to 80 °C and from 25 °C to 150 °C indicating
that the nanocomposite can
function as a very reproducible thermal sensor.
(a)
(b)
Figure 2.8 (a) Schematic showing chromaticity (chroma)
distribution from gray (dull)
color at the center to saturated (vivid) color at the perimeter
(arrows indicate chromatic
transition temperatures discussed in the text); (b) Chromaticity
versus temperature plots for
poly-TCDA and poly-TCDA/ZnO composites of three different
compositions.
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32
(c)
(d)
Figure 2.8 (c) Chromaticity of poly-TCDA/ZnO (5wt%) as a
function of thermal cycle;
(d) Chromaticity of poly-TCDA/ZnO (15wt%) as a function of
thermal cycle. (Continued)
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33
2.4 Conclusions
Raman, FTIR, DSC and colorimetric measurements have been used to
understand the
thermochromic reversibility introduced by composite formation of
poly-TCDA with ZnO
in the particle size range below 100 nm. Raman frequency
upshifts occur at 70 °C and
120 °C in pure poly-TCDA and poly-TCDA/ZnO composites,
respectively, corresponding
to chromatic transitions. The peak shifts of the Raman-active
υ(C≡C) and υ(C=C)
vibration peaks increase with increase of ZnO content.
Poly-TCDA/5 wt% ZnO shows
only partially reversible color change, whereas poly-TCDA/10 wt%
ZnO and
poly-TCDA/15 wt% ZnO change color reversibly and have similar
thermochromic
responses. The Raman data indicate the irreversible formation of
an amorphous
poly-TCDA phase in poly-TCDA/5 wt% ZnO but not in poly-TCDA
composites with 10
wt% and 15 wt% ZnO. Chelate formation between ZnO and neigboring
side chain -COOH
head groups is proposed which leads to reversibility of the
chromatic transition and
increase of the chromatic transition temperature. Compared with
the results of previous
study on PCDA, the amorphous feature can be found in poly-TCDA
with low
concentration of ZnO exclusively, which probably results from
the fact that the carbon
chain in TCDA is shorter than that in PCDA. Excellent
reversibility in chromaticity as a
function of number of cycles from 25 °C to 80 °C and from 25 °C
to 150 °C is observed
indicating that the poly-TCDA/ZnO nanocomposites can function as
a temperature sensor.
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34
CHAPTER 3
REVERSIBLE CHROMATIC SENSOR FABRICATED BY INKJET PRINTING
TCDA-ZINC OXIDE ON A PAPER SUBSTRATE
3.1 Introduction
Recently, great attention has been drawn to printing and
deposition of functional materials
on paper substrates because the features of paper substrates
could enable their use in
flexible, light-weight and disposable devices115,116
. Various organic and inorganic
conducting, semiconducting, and dielectric materials for
applications in displays, sensors,
energy storage materials, and memory devices on paper substrates
have been successfully
prepared and widely reported95,117-121
. Among the patterning methods employed for
deposition of functional materials on paper substrates, the
inkjet printing method122
is of
great interest due to the method’s well-known
attributes91-93
which are mentioned before.
As far as sensing materials are concerned, polydiacetylenes
(PDAs) have been
widely studied as a chromatic sensor material because they can
respond to a variety of
signals, such as mechanical, temperature and chemical
stimuli2-6
. Solid state topotactic
photo-polymerization of diacetylene monomers by exposure to UV-
or γ-radiation and
subsequent thermochromism in closely packed and uniformly
ordered thin films of various
PDAs are well known123
and have been widely studied for temperature-sensing
applications.
PDAs have a one-dimensional conjugated backbone with a strong π
to π*
absorption band in the red spectral region of the optical
spectrum which gives rise to an
intense blue color in the polymer. The blue phase undergoes a
temperature-induced or
thermochromic transition observed in many PDAs to a red phase on
heating. The blue to
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35
red chromatic transition is either irreversible or reversible
under heating and cooling cycles
depending on the chemical structure and interactions on the side
chains of the PDA.
PDAs prepared from 10, 12-pentacosadiynoic acid (PCDA) and
10,
12-docosadiynedioic acid (DCDA) have been widely
investigated110,124,125
, and a
PCDA-based ink has been inkjet printed by Bora Yoon et al94
. Yoon’s aqueous PCDA
requires a surfactant to maintain an acceptable concentration
for inkjet printing without
aggregation. Belonging to the same group of PDA monomers and
sharing a similar
molecular structure but smaller molecular weight as PCDA, TCDA
has not been given as
much attention for chromatic sensor or for inkjet printing
applications. Also, according to
the work done by Patlolla et al113
, PCDA-metal oxide nanocomposites has shown the effect
of nanoscale metal oxides on changing the chromatic properties
of poly-PCDA. Inkjet
printing would provide another fast method in ionic bond
strengthened PDA thin film
fabrication. In this paper attention is given to: (a) Inkjet
printing of relatively high
concentration TCDA-ZnO suspension without using surfactant on a
paper substrate, and
(b) The thermochromic properties of materials fabricated by
inkjet printing.
3.2 Experimental Section
3.2.1 Materials
TCDA was purchased from GFS Chemicals and nanocrystalline ZnO
(99%) was purchased
from Sigma-Aldrich and used without further purification.
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36
3.2.2 Preparation of TCDA and TCDA-ZnO Composites Ink
TCDA was purified by dissolving and removing the polymerized
solid. TCDA composite
inks were prepared by suspending a nominal amount of 5wt% ZnO in
TCDA/chloroform
solution with the ratio of TCDA/chloroform equal to 0.1mol/50ml.
The suspension was
sonicated in a water bath at room temperature for 15 min and
then allowed to stand for 1
hour to enable removal of unsuspended ZnO. It was estimated that
the final suspension
contained approximately 2.5 wt% ZnO in TCDA.
3.2.3 Design and Fabrication of Poly-TCDA Based Chromatic
Sensor
The design and fabrication the poly-TCDA based sensor is
conducted using a Fujifilm
Dimatix printer model DMP-2800, which is based on piezoelectric
inkjet technology. The
cartridge with a nozzle pore size of ca. 20 μm in diameter was
filled with a
TCDA/chloroform solution or suspension of the TCDA/ZnO in
chloroform and the ink
was printed on unmodified A4-sized paper. Both TCDA and TCDA/ZnO
were inkjet
printed with 20 volts applied on nozzle pores, the nozzle
cleaning was carried after every 5
bands of printing. After inkjet printing either TCDA or TCDA/ZnO
composite suspensions
on flexible substrates, the printed images were formed following
solvent evaporation at 40
°C. The patterns for Raman and optical densitometry measurements
were in 5mm × 5 mm
square shapes.
3.2.4 Synthesis of Poly-TCDA-ZnO Nanocomposites
The TCDA and TCDA/ZnO composites inkjet printed on substrates
were polymerized to
the blue phase of poly-PCDA composites by irradiating with a 254
nm wavelength UV
source after inkjet printing. Figure 3.1 illustrates the
polymerization reaction of TCDA
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37
under UV exposure. Red phase poly-TCDA was prepared by heating
up the inkjet printed
pattern to the chromatic transition temperature.
Figure 3.1 Structure of TCDA (left) and poly-TCDA after
UV-irradiation (right).
3.2.5 Material Characterization Techniques
Room temperature Raman spectra of thin films fabricated by
inkjet printing were obtained
primarily by using a Mesophotonics Raman spectrometer with 785
nm laser excitation.
Temperature-dependent Raman measurements for the inkjet printed
patterns were carried
out with an EZ Raman-L system (LE-178155, Enwave Optronics, Inc)
coupled to a Leica
optical microscope. The spectrometer was calibrated using
silicon wafer and diamond
powder standards to a frequency accuracy of 1 cm-1
. The variable temperature optical stage
used is from Linkam Scientific Instruments Ltd. Thin films for
the Raman measurements
were prepared by 5-layer inkjet printing the suspensions of
TCDA/ZnO in chloroform on a
silicon wafer. After 254 nm uv-radiation, the polymerized TCDA
and poly-TCDA-ZnO
were measured directly.
Fourier Transform Infrared (FTIR) was carried out using a
Nicolet ThermoElectron
FTIR 560 spectrometer with a MIRacle attenuated total
reflectance (ATR) platform
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38
assembly and a Ge plate. Poly-TCDA powder was obtained by
scratching off the inkjet
printed poly-TCDA/composites on a Kapton film. The inkjet
printing parameters on
Kapton were the same as that for inkjet printing on paper
substrates.
Chromaticity, which is a quantitative measure of the vividness
or dullness of a
color (or how close the color is to either the gray or pure hue)
was measured directly on
printed films using an X-Rite 518 optical densitometer as the
film was heated on a
temperature-controlled hot plate.
A Mettler Toledo DSC instrument with a FP90 central processor
was used to obtain
the DSC data of inkjet printed precursor, polymer and
composites. Measurements were
made on 10 mg powder wrapped in a small disk with aluminum foil
under
heating/cooling/heating cycles in the temperature range from 25
°C to 300 °C at a rate of 10
°C min-1
.
3.3 Results and Discussion
In our previous work126
, we studied the poly-TCDA and poly-TCDA/ZnO powders by
using Raman, ATR-FTIR, DSC methods. The main conclusion from
that work was that
ZnO can form a chelate with neighboring side chain -COOH head
groups of poly-TCDA,
which resulted in reversible chromatic transition and increase
of the chromatic transition
temperature126
. In this work, we characterize the solid phase of inkjet
printed films. The
major goal of the characterization of inkjet printed films is to
confirm that the printing
process does not change the functionality of the poly-TCDA and
poly-TCDA-ZnO films.
-
39
3.3.1 Feasibility of Inkjet Printing TCDA and TCDA-ZnO
Composites
Figure 3.2 demonstrates that, in actuality, the ink is not
visible when it is in the monomer
state because TCDA does not absorb visible light (Figure 3.2a).
However, the
polymerization of TCDA initiated by UV-irradiation (254 nm, 1
mW/cm 2, 30 s) results in
the formation of blue image patterns (Figure 3.2b). This
observation supports the proposal
that PDA monomers are well-aligned and closely packed following
printing and that PDAs
are indeed generated on the paper substrate. This is an
important result because if the
closely packed alignment of the PDA monomers were disrupted
during the printing and
fixing steps, polymerization would not proceed.
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40
(a)
(b)
Figure 3.2 Digital photographs of inkjet printed TCDA: (a) TCDA
monomer before UV
radiation; (b) TCDA after UV radiation.
3.3.2 Raman and ATR-FTIR Spectroscopy of Poly-TCDA and
Poly-TCDA-ZnO Composites
The molecular structural changes of the chromatic transition and
molecular interactions on
nanocomposite formation were studied by ATR-FTIR and Raman
spectroscopy at room
temperature in both the red and blue phases for pure poly-TCDA
and for the blue phase in
poly-TCDA-ZnO.
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41
(a)
(b)
Figure 3.3 ATR-FTIR spectra at room temperature of: (a) Pure
poly-TCDA in the blue and
red phases; (b) Poly-TCDA and poly-TCDA-ZnO in the blue phase
between 700 and 3300
cm-1
.
-
42
(c)
Figure 3.3 ATR-FTIR spectra of: (c) Poly-TCDA and poly-TCDA-ZnO
in the blue phase
expanded in the 750 and 1900 cm-1
spectral range. (Continued)
Figure 3.3a shows the ATR-FTIR spectra of poly-TCDA in its blue
and red phases,
and Figure 3.3b and 3c show the spectra of poly-TCDA and
poly-TCDA-ZnO in the
frequency region from 700 to 3300 cm-1
, and in the expanded range from 750 to 1900 cm-1
,
respectively. As in our previous work 126
, lines at 2920 and 2847cm-1
are assigned to the
asymmetric and symmetric stretching vibrations, respectively, of
the CH2 groups of the
hydrocarbon side chains on poly-TCDA, and those at 1463, 1417
and 1694 cm-1
can be
attributed to the CH2 scissoring and hydrogen-bonded carbonyl
C=O stretching vibrations,
respectively. On comparing the FTIR spectra of pure poly-TCDA
with that of
poly-TCDA-ZnO shown in Figure 3.3b and 3.3c, it is observed that
a relatively strong line
appears at 1540 cm-1
in the spectrum of poly-TCDA-ZnO together with a decrease in
intensity of the C=O stretching line at 1694 cm-1
. The 1540 cm-1
line (indicated by an arrow
in Figure 3.3c) can be assigned to an asymmetric COO- vibration
and its presence in the
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43
spectra indicates the formation of a chelate between the side
chain -COOH head groups of
poly-TCDA and Zn2+
from ZnO, which is similar to the result reported by Patlolla et
al113
.
785nm laser excited Raman spectra are obtained to probe the
resonance-enhanced
molecular vibrational modes of the conjugated polymer backbone.
From the Raman
spectra in Figure 3.4 and Table 3.1 for pure poly-TCDA, two
primary lines at 2083 cm-1
and 1455 cm-1
are observed at room temperature in the blue phase, which can be
clearly
assigned to the C≡C and C=C stretching modes of the polymer
backbone, respectively. In
the red phase at 25 °C, the C≡C and C=C stretching vibration
frequencies occur at 2118
cm-1
and 1516 cm-1
, respectively. Compared with those in blue phase, the upshift
in
frequency is due to the irreversible stress on the polymer
backbone caused by the breakup
of the head group hydrogen bonds in the red phase.
Figure 3.4 785 nm laser-excited Raman spectra of the inkjet
printed blue (bottom) and red
(top) phases of poly-TCDA at room temperature.
Figure 3.5 shows the Raman spectrum of pure poly-TCDA in the
blue phase
compared with the blue phase spectra of poly-TCDA-ZnO composites
prepared by the
inkjet printing method. From Figure 3.5, it is evident that a
very weak line at 2257 cm-1
in
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44
the C≡C stretching mode region of poly-TCDA shows up in the
Raman spectra of
poly-TCDA-ZnO, which can be assigned to a diyne formed as a
defect on the backbone
due to the interaction between TCDA and ZnO 114
. Similar to the poly-TCDA prepared by
the conventional method in previous work, the line at 1516
cm-1
shows up in pure
poly-TCDA fabricated by inkjet printing, which can be attributed
to the presence of a red
phase impurity in the majority blue phase. Another feature which
is similar to the previous
work 126
in Figure 3.5 is that the line at 690 cm-1
and the triplet of lines at 1250 cm-1
assigned above to polymer backbone modes, show substantial
decrease in intensity with
composite formation; meanwhile a broad diffuse scattering
appears around the line at 690
cm-1
. These spectral effects above could be due to the increase of
the degree of long-range
disorder caused by the formation of a chelate between ZnO and
C=O groups, and the
disordered molecular arrangement reduces resonance interaction
with the polymer
backbone. By comparison with poly-TCDA (see Table 3.1), the
Raman frequency upshift
of the C≡C and C=C backbone stretching vibrations in red phase
decreases in the presence
of ZnO, which suggests that the backbone stress is lowered due
to the interaction of ZnO
with poly-TCDA.
Table 3.1 C≡C and C=C Raman Peak Frequencies in Pure Poly-TCDA
and in
Poly-TCDA-ZnO Nanocomposites in the Blue and Red Phases Phase
Poly-TCDA, 25°C Poly-TCDA/ZnO (2.5 wt%) [Blue,25°C; Red, 150°C]
υ(C≡C) cm-1 υ(C=C) cm-1 υ(C≡C) cm-1 υ(C=C) cm-1
Blue 2083 1455 2081 1453
Red 2118 1516 2108 1507
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45
Figure 3.5 Raman spectra of pure poly-TCDA and poly-TCDA-ZnO
thin film fabricated
by inkjet printing.
The analysis of ATR-FTIR and Raman spectra further proves that
inkjet printing
does not affect the close packing alignment of the TCDA molecule
and demonstrates the
feasibility of polymerization after TCDA was inkjet printed on a
paper substrate. Besides,
ATR-FTIR and Raman spectra indicate the interaction between TCDA
and ZnO.
Compared with poly-TCDA and poly-TCDA-5 wt%ZnO powders prepared
by the
conventional method, no obvious spectral differences are
observed.
3.3.3 Temperature-Dependent Raman Spectroscopy of Poly-TCDA and
Poly-TCDA-ZnO Composites
Temperature-dependent Raman spectroscopy is used to further
investigate the
thermochromism of poly-TCDA or poly-TCDA-ZnO composites. Raman
spectra under
heating and cooling cycles in the 25 °C to 150 °C temperature
range for poly-TCDA and
poly-TCDA-ZnO are shown in Figures 3.6 and 3.7, respectively.
The Raman data were
taken in steps of 10 °C from 30 °C to 150 °C and also recorded
in 10 °C steps during the
cool down to room temperature.
-
46
(a)
(b)
Figure 3.6 785 nm laser excited Raman spectra of pure poly-TCDA
as a function of:
(a) Increasing temperature, and (b) Decreasing temperature.
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47
(a)
(b)
Figure 3.7 785 nm laser excited Raman spectra of poly-TCDA-ZnO
(2.5 wt%) as a
function of: (a) Increasing temperature, and (b) Decreasing
temperature.
Figure 3.7 shows the variable temperature Raman spectra of
poly-TCDA-ZnO (2.5
wt%). In contrast to the variable temperature spectra for pure
poly-TCDA (Figure 3.6), a
broad scattering band at 690 cm-1
appears in the spectra with increasing intensity as the
temperature is raised to form the red phase. Together with the
ATR-FTIR results, it could
be due to the the C=O group of TCDA forming a COO- ion with ZnO.
The ionic bonding
formed however is not strong enough to maintain the backbone
structure of poly-TCDA
-
48
under thermal stress conditions. The irreversible property
caused by lack of strong enough
ionic bonding is supported by the fact that the variable
temperature Raman spectra show
the same intensity and no obvious wavenumber shifts for the C≡C
and C=C modes on
cooling, which is in agreement with the data for poly-TCDA-ZnO
(5 wt%) reported
previously126
. The wavenumber changes for the C≡C and C=C modes as a function
of
temperature is shown in Figure 3.8.
Figure 3.8 C≡C and C=C stretching mode frequencies versus
temperature for inkjet
printed poly-TCDA and poly-TCDA-ZnO as a function of
temperature.
3.3.4 Differential Scanning Calorimetry (DSC) Measurements
DSC measurements were performed to provide further understanding
of the nature of the
interaction between TCDA/poly-TCDA and ZnO. DSC data were
obtained for pure TCDA
monomer, poly-TCDA, and poly-TCDA-ZnO, at heating and cooling
rates of 10 °C min-1
between 25 °C and 300 °C. The heating scan for pure TCDA in
Figure 3.9a shows an
endothermic peak at 61 °C due to melting. On cooling scan, a
down-shifted exothermic
crystallization peaks at 59 °C due to hysteresis is observed.
The heating scan for
poly-TCDA in Figure 3.9b shows an endothermic peak at 61 °C due
to melting of the
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49
unpolymerized monomer. A broad endotherm with a shoulder at 154
°C and a peak 190 °C
are assigned to the melting of poly-TCDA. On cooling, polymer
crystallization is indicated
by broad exothermic features at 159 °C and 194 °C which are
upshifted due to hysteresis
relative to the corresponding endothermic melting peaks.
Crystallization of unpolymerized
monomer is not observed during the cooling cycle probably due to
loss of the monomer by
sublimation during thermal cycling. The heating scan for
poly-TCDA-ZnO in Figure 3.9
shows an endotherm at around 57 °C due to unpolymerized monomer
and a new
endothermic feature at 132 °C due to melting of the monomer
modified by the chelate
formation discussed above, the broad exothermic features between
159 °C and 209 °C
which could refer to that of poly-TCDA are assigned to the
melting of poly-TCDA. The
new endothermic peak in poly-TCDA-ZnO is consistent with the
FTIR and
temperature-dependent Raman spectra discussed above suggesting
an interaction of ZnO
particles with the head group of the polymer side-chain to form
a chelate which can be
schematically written as: Zn2+
(COO-)2. The temperature dependent Raman and DSC
results also suggest the inkjet printing does not affect the
interaction between TCDA and
ZnO. It was also observed that the ZnO particles are uniformly
distributed after deposition
on the substrates.
-
50
Figure 3.9 DSC heating scans for: a) TCDA, b) poly-TCDA, c) ZnO
(
-
51
between Zn2+
and the head group of TCDA to assist in the thermal-stress
release, reversible
color change of poly-TCDA-ZnO could be limited at a particular
temperature. According
to the temperature-dependent Raman spectra and
temperature-dependent chromaticity
plots, 80 °C could be the suitable temperature for reversibility
of the chromatic transition.
Figure 3.10 Chromaticity versus temperature plots for poly-TCDA
and poly-TCDA-ZnO
(2.5 wt%) composite inkjet printe